I believe in moderation. I think going to extremes, whether with restricting food or with overdoing exercise, can be counterproductive.
Too much exercise just breaks down muscle without giving the body time to recover and rebuild. And overly restricting food intake, especially not getting enough protein, makes things worse in this way.
During my first round of weight loss, I found myself feeling really tired after a few months. I was exercising 5-6 days a week, with walking for 30 minutes, cardio level for 30 to 40 minutes, and weights for 20 minutes, alternating body areas every other day. 3FC member Meg suggested that I increase my protein--and so I did, and I felt a lot better.
People want to lose weight fast, so they think if they work harder and restrict more, they will lose faster. IMO it's better to (1) have patience (2) not treat the body like a machine.
Don't worry about letting up a little and making adjustments. You won't be "struck fat"!